As a system that measures particle concentration in exhaust gas emitted from a diesel engine, for example, as disclosed in JPA 2008-164419, there is a system that introduces exhaust gas into a particle counting device such as a CPC to count the number of particles.
Meanwhile, the exhaust gas is emitted in a high temperature state (e.g., 500° C.), and therefore in order to measure particle concentration in the exhaust gas, temperature of the exhaust gas should be reduced to a measurable temperature of the particle counting device. For this purpose, a possible method is that, by circulating the exhaust gas in the high temperature state in a pipe, the exhaust gas is cooled by thermal conduction of the pipe. However, this method has a problem that particles in the exhaust gas are attached to an inner wall of the pipe by thermal phoresis and the like, and consequently the number of particles in the exhaust gas introduced into the particle counting device is reduced to give rise to a measurement error.
For this reason, the conventional particle measurement system is, on an upstream side of the particle counting device, provided with a diluting device that dilutes sampled exhaust gas with low temperature diluting air (see FIG. 4 of JPA 2008-164419). On the basis of this, the particle measurement system is configured to reduce temperature of the exhaust gas to the measurable temperature of the particle counting device while preventing the particles from being attached to the inner wall of the pipe by the thermal phoresis and the like.
However, in the case where the particle measurement system is configured to be provided with the diluting device, the diluting device, peripheral devices (mass flow controller, pressure sensor, on/off valve, and the like), and pipes are required to cause problems that the system is increased in size, complexity, and cost. Also, by making the diluting device intervene, control of a diluting ratio, or the like, becomes a new error factor. Further, the system is increased in complexity, and therefore maintenance work on the system is also complicated.